Mock That Monkey

patch_games.py

In [42]: class Foo(object):
   ....:     def bar(self):
   ....:         return 'x'
   ....:

In [43]: class Baz(object):
   ....:     def biz(self):
   ....:         return Foo().bar()
   ....:

In [44]: Baz().biz()
Out[44]: 'x'

In [45]: with mock.patch.object(Foo, 'bar', return_value='y') as fu:
    print(Baz().biz())
   ....:
y